The present invention is a variation of the "HUBNET" network which itself is a packet switching network. HUBNET is a packet-switching local area network utilizing optical fibers as the transmission medium. This network and the protocols utilized therein are thoroughly described in the literature. For example, see E. Stewart Lee and Peter I.P. Boulton, "The Principles and Performance of HUBNET: A 50 Mbit/s Glass Fiber Local Area Network," Vol. Sac/1 IEEE J. Selected Areas In Communications (No. 5, Nov. 1983), incorporated in its entirety herein by reference. The CMSA/CD protocol is described in the Metcalfe et al U.S. Pat. No. 4,063,220 and the Lo U.S. Pat. No. 4,539,677, both incorporated herein by reference. The CSMA/CD protocol sets the operating conditions and data format for a multiuser common bus.
The network structure of HUBNET is a pair of trees rooted at a central hub, one tree being used for selection and the other tree being used for broadcast. Intelligent devices called subhubs form the internal nodes in the tree and conventional devices called Network Access Controllers (NACs) in the leaves of the tree provide the interface for communication between the terminals attached to them and to the network. The trees are matching in every respect and the HUBNET network is depicted in FIG. 1.
The communication medium in HUBNET is twin-fiber, one fiber being used for transmission and the other fiber being used for reception. While the HUBNET proposal utilizes selection nodes and broadcast nodes that are constructed together, the two nodes remain almost completely independent. Furthermore, the internal nodes, or subhubs, in each tree are technically similar to the main hub. All communications between devices must first travel up the selection tree to the selection hub, then must cross over to the broadcast hub; and finally must travel to the destination device through the broadcast tree.
A packet of data to be transmitted is sent in a frame having a predetermined format. The protocol is described in the Lee and Boulton article and for the purpose of completeness a packet is depicted in FIG. 2. When a device in the HUBNET network transmits, the selection side of the network takes over and the frame is sent all the way to the root of the selection tree to the main hub. At the main hub from the selection side the frame is linked to the central hub of the broadcast tree and then, the frame is broadcast to all of the NAC receivers along the branches of the broadcast tree, including the station which transmitted the message. Thus, this broadcast also serves as an echo signal to the source NAC. However, if at any point on the selection side, a subhub is already committed to a previous frame of data, the new incoming frame is ignored. In addition, the source, not bearing the echo after a fixed time, is programmed to repeat the transmission of the frame. Thus, an NAC must wait until an echo signal is received at its reception side before it can transmit a new packet of information. This leaves much room for large transmission delays.
Although the HUBNET is one of the best known and best documented fiber-optic networks, other structures have been used and are known. For example, the ETHERNET network is a bit-serial receiver-transmitter network that is continuously connected to all communicating devices along a single path that has terminators at each end. The ETHERNET network is described in the Metcalfe et al U.S. Pat. No. 4,063,220 and in the article by that inventor referenced in the Lee and Boulton article, supra. However, the ETHERNET system has been criticized as not being suitable for use with glass fiber as a communication medium primarily because of the large power drop that must be accommodated in a glass fiber network.
Another common topology is the ring or closed loop topology. Examples of this topology in an optical fiber network are disclosed in the Kao U.S. Pat. No. 4,017,149 and the Herskowitz U.S. Pat. No. 4,366,565, both incorporated herein by reference. This topology has been used for fiber optical bus communication systems. The disadvantage, however, of this topology is that only one station can talk at a time, thereby tieing up communications between other stations to either the communicating station or to another station not in the "conversation." Furthermore, this network is somewhat limited in the number of devices that can be attached.
Another well known topology is the star network. In a star, all devices must communicate directly with a central site. An example of a star network is the FIBERNET network in which up to 19 devices are connected by optical fibers to a central hub. Another example is depicted in the Svensson U.S. Pat. No. 4,553,235, and the Usui U.S. Pat. No. 4,531,239, both incorporated herein by reference. The star network has the disadvantage of requiring a large amount of extra cabling and the tangling of the cabling at the central site. Furthermore, damage to the central hub will completely deactivate the entire network.
All network communications systems that utilize optics in the transmission of the messages are comprised also of a plurality of optical connecting means, such as optical fibers, optical couplers, electro-optical converters, and optical-electrical converters. All of this optical hardware is conventional and commercially available. It is also described in the literature, such as in the Ozeki et al U.S. Pat. No. 4,511,208, incorporated herein by reference and the previously mentioned Herskowitz patent. Although the Ozeki et al patent is primarily directed to optical couplers, it also discloses a plurality of fiber optic communication networks, such as the T-coupler network, the star-coupler network, the plural star coupler network that has a plurality of sub-star couplers directly coupled to one main star coupler and the plural star-T coupled network. Generally, most conventional optical communication networks utilize two fibers in the transmission lines, one fiber for communications in each direction. However, much work is currently being done on utilizing a single optical fiber and eventually a single optical fiber transmission line may become common.
Local area networks using multiple transmission techniques must account for the possibility of a collision of data, that is two different stations sending data simultaneously. The CSMA/CD protocol is a common protocol used for dealing with this possibility. Under the protocol, a station ready to send a prepared data frame first checks the network bus for an idle condition and when detected immediately sends the message. The stations simultaneously listen to the bus and compare the data being transmitted with the data received. When a collision or interference condition is detected, the station aborts the transmission. Aborted transmissions are rescheduled by the stations after "backoff" delay intervals that are randomly selected, thereby lessening the likelihood of repeated collisions by the same competing stations.
All of these prior art networks have disadvantages or features that do not make them compatible for use in a fiber-optic network. It is desirable to have a network that not only permits future expansion, but also allows local communications so that the entire network is not tied up, is relatively inexpensive, can account for the power drops in the transmission of a fiber optic transmission line, and is redundant enough so that a single casualty does not disable the entire network.